Chains & cassettes are always new (though I’ve bought unused s/h off here before).
Tyres are usually new as well, though I’ve acquired 20 or so with secndhand bikes I’ve bought, so I’ll not be buying nay for a while.
Frames – usually secondhand (though I bought a new 14″ On-One for one of my kids ‘cos it was £90)
Wheels – always secondhand, though if I kill a rim I put a new one on
Seatpost/stem/bars – secondhand
Pedals – Always new SPDs now – so cheap that it’s not worth buying knackered old stuff
Forks – always seciondhand thenserviced and set up for me
Shocks – if I can find a coil shock to fit, then secondhand
Saddle – depends
BB – if there’s an unused secondhand one, fine, but otherwise new
Cranks – look for a bargain then buy new